How do I change a password that's hashed and salted in MVC 5?

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C#

SQL

MVC

MVC5

SQL-Server-2014

Hi,

I'm creating a Homepage for a website and I've stumbled in a small feature.

So I'm using a database (SQL) to save my users.
I'm throwing some alt and hashing their passwords but then my problem comes.

If the user attempts to change his password,
I ask him to insert his older password, his new and confirm his new.

Yet since the passwords are hashed and salted they don't match and my stored procedure in SQL returns -2.

ALTER PROCEDURE [dbo].[spChangePassword]
(
        @sUsername varchar(50),
	@sPasswordNew varchar(100),
	@sPasswordNewSalt varchar (128),
	@sPasswordNew varchar (100),
	@sPasswordNewSalt varchar (128),
)
AS
BEGIN
	SET NOCOUNT ON;
	
    if (exists (select 1
                from USERS
	        where Username = @sUsername 
                and Password = @sPasswordOld))
				
    begin
		if (exists (select 1
					from USERS
					where Username = @sUsername
					and Password != @sPasswordNew))

			begin
				select 1;
				 
				update BLC_USER
				set Password = @sPasswordNova,
				Password_Salt = @sPasswordNewSalt,
				where Username = @sUsername;
			end	

		 else
				select -1; -- New Pass = Old Pass, please chnage
	end
	else
		select -2; -- Old Pass is wrong
END

Am I doing something wrong in regards to the hashing passwords?
How can I compare two salted passwords?

Cheers,
Zamuk